252
HILARY B. MOORE
We have assembled available published and some unpublished data
on commencement of spawning of marine invertebrates. These were
grouped in 6OC assemblages of annual mean temperatures. In records
from the southern hemisphere, six months have been added to the
breeding dates. Figure 31 displays the results with polar conditions in
the center and the tropics at the periphery. The months progress
clockwise as shown. The well documented shift from summer spawning
in cold waters to spring spawning in temperate waters is clear. What is
striking, though, is the return shift t o initiation of spawning in the
summer in the tropics. A plot of the percentage of species spawning in
each month yields a similar pattern, but with some time lag.
To make a comparable curve for temperature rise we took the
inshore data from the Coast and Geodetic Survey publications for both
American coasts. We assumed that the normal seasonal temperature
varied on a sine curve, and for each seasonal mean temperature and
seasonal range computed the number of days after the date of winter
minimum when a rise of 2OC and 4OC would have occurred; these are
plotted in Fig. 32. The spawning data are worldwide, and the American
temperature data are more restricted, but their general trend is surely
applicable for purposes of comparison. It is clear that in polar waters
it would be necessary to wait until the summer for there to be a 4°C rise
in temperature. In temperate waters this would have occurred in the
spring, while in the tropics a delay until summer would also be necessary.
The time shift in spawning from polar to temperate waters has generally
been related to the season of abundance of the phytoplankton which
the larvae need for food. This may well be true but would not account
for the shift to summer breeding in the tropics where phytoplankton
tends to be present at a rather constant level throughout the year.
Nor is it a very satisfactory explanation in polar seas where so few
species produce planktonic larvae. Species breeding throughout the
year were excluded from the calculations on date of commencement of
spawning, but included in the ones on percentage of species breeding
each month.
Despite such studies as that of Stephenson (1934) the belief still
lingers that tropical animals breed throughout the year. It is true
that there are some species breeding at all times, but most species have
a limited spawning season, and some a season as restricted as any in
polar seas. Figure 33 shows the frequency distribution of duration of
spauming and how this varies with temperature. There is a clear
general trend towards a more prolonged spawning period in warmer
waters, and there is also an increase in the proportion of species which
spawn throughout the year, but even in the highest temperature group
HILARY B. MOORE
We have assembled available published and some unpublished data
on commencement of spawning of marine invertebrates. These were
grouped in 6OC assemblages of annual mean temperatures. In records
from the southern hemisphere, six months have been added to the
breeding dates. Figure 31 displays the results with polar conditions in
the center and the tropics at the periphery. The months progress
clockwise as shown. The well documented shift from summer spawning
in cold waters to spring spawning in temperate waters is clear. What is
striking, though, is the return shift t o initiation of spawning in the
summer in the tropics. A plot of the percentage of species spawning in
each month yields a similar pattern, but with some time lag.
To make a comparable curve for temperature rise we took the
inshore data from the Coast and Geodetic Survey publications for both
American coasts. We assumed that the normal seasonal temperature
varied on a sine curve, and for each seasonal mean temperature and
seasonal range computed the number of days after the date of winter
minimum when a rise of 2OC and 4OC would have occurred; these are
plotted in Fig. 32. The spawning data are worldwide, and the American
temperature data are more restricted, but their general trend is surely
applicable for purposes of comparison. It is clear that in polar waters
it would be necessary to wait until the summer for there to be a 4°C rise
in temperature. In temperate waters this would have occurred in the
spring, while in the tropics a delay until summer would also be necessary.
The time shift in spawning from polar to temperate waters has generally
been related to the season of abundance of the phytoplankton which
the larvae need for food. This may well be true but would not account
for the shift to summer breeding in the tropics where phytoplankton
tends to be present at a rather constant level throughout the year.
Nor is it a very satisfactory explanation in polar seas where so few
species produce planktonic larvae. Species breeding throughout the
year were excluded from the calculations on date of commencement of
spawning, but included in the ones on percentage of species breeding
each month.
Despite such studies as that of Stephenson (1934) the belief still
lingers that tropical animals breed throughout the year. It is true
that there are some species breeding at all times, but most species have
a limited spawning season, and some a season as restricted as any in
polar seas. Figure 33 shows the frequency distribution of duration of
spauming and how this varies with temperature. There is a clear
general trend towards a more prolonged spawning period in warmer
waters, and there is also an increase in the proportion of species which
spawn throughout the year, but even in the highest temperature group